1. Field of the Invention
This invention relates to an improved method for plasma cleaning of an aluminum surface and, more specifically, it relates to a rapid, effective means for employing a cold plasma process containing a preferred blend of oxygen and freon to effect such cleaning and provide other benefits.
2. Description of the Prior Art
It has been known to employ low temperature, gaseous, plasma reactor apparatus wherein a sample is placed in a chamber which under desired conditions is subject to radio frequency power to excite the carrier gas molecules and charge some of them into ions or excited gas molecules, such as those of oxygen. The gaseous plasma is highly reactive and can cause low temperature-combustion of the organic materials or impurities in the sample. See, generally, U.S. Pat. Nos. 3,875,068; 3,879,597; and 4,362,632. Among the materials on which such a system has been employed are semi-conductors which may be etched by this plasma means. See U.S. Pat. No. 4,138,306.
It has also been known to use a wide variety of gases as the carrier gas. Included within this group are, for example, oxygen, nitrogen, hydrogen, ammonia, helium, argon, and freon. See U.S. Pat. Nos. 3,875,068; and 4,442,338.
It has also been known to suggest using a mixture of oxygen and freon in the treatment of semi-conductors within a plasma generating station. See U.S. Pat. No. 4,138,306.
U.S. Pat. No. 4,357,203 discloses oxygen plasma etching of a polyamide film followed by second plasma etching employing a mixture of argon and hydrogen.
U.S. Pat. No. 4,442,338 discloses plasma etching apparatus employed for etching aluminum and aluminum alloys. The etching is effected by means of a chlorine-containing gas. After etching, thermal treatment is effected by a gas heated to 40.degree. C. to 200.degree. C. which may include argon, helium, neon, nitrogen, oxygen or hydrogen, or a blend thereof.
U.S. Pat. No. 4,529,474 discloses the cleaning of the inside walls of a reaction chamber employing a mixture of carbon tetraflouride and oxygen with the former being present in the mount of 8 to 30 percent. The apparatus being cleaned is one used to clean the apparatus employed in forming a deposited film by a plasma chemical vapor deposition process. It is the reaction product that is attached to the inside walls of the reaction chamber during this CVD process that is cleaned by the gas mixture disclosed in this patent.
U.S. Pat. No. 4,370,195 discloses removal of plasma etching residues using a nitrogen glow discharge for a period long enough to either remove the residues or render them susceptible to removal by oxygen plasma or organic solvents.
U.S. Pat. No. 3,957,608 discloses a method for removing surface oxidation of aluminum in a vacuum of from 10.sup.4 to 10.sup.-3 Torr. Oiled aluminum coated sheet steel is passed through a plasma reactor having an argon and oxygen gas mixture. The reactor also contains a block of pure aluminum. The action of the plasma is said to release the aluminum atoms from the block and have them combined with the oxygen to form alumina which is deposited on the sheet to produce a continuous alumina coated sheet.
Despite the foregoing disclosures, there remain a real and substantial need for a method of cleaning an aluminum surface in a rapid and efficient manner.